An operational amplifier is an analog circuit that amplifies voltages with a high gain. It produces as an output, a voltage proportional to a voltage difference on two inputs, i.e. the inverting and non-inverting inputs. Operational amplifiers are generally manufactured in integrated circuit form. In a typical layout, several separate operational amplifiers may be integrated on one and the same substrate with numerous other circuit elements, both analog and digital.
A simplified diagram of an exemplary operational amplifier 1 of the CMOS type is represented in FIG. 1. It therefore comprises two inputs 2, 3 to which are respectively applied voltages Vp, Vm, and an output 4 at the amplified potential Vout. The example is based upon an architecture with three stages comprising a differential input stage 6, in general comprising NMOS or PMOS transistors or a combination of the two, a drive stage 7, and a power stage 8. The latter constitutes also the output stage of the operational amplifier. FIG. 2 illustrates in greater detail the internal electrical circuit of an operational amplifier comprising two stages. The first differential input stage 6 comprises a pair of transistors M1, M2 and a current source Ib.
An operational amplifier 1 amplifies the voltage difference between its inputs 2, 3, and the output voltage Vout is in theory in the middle of the output dynamic range when these inputs are at the same potential (that is to say when Vp=Vm, for example, by being linked directly to one another). However, as is known, in practice an operational amplifier exhibits a spurious output voltage, called the offset voltage or more simply the “offset.” This offset voltage, which varies with temperature, results from an imbalance between the characteristics of the inputs of the amplifier and adds an inaccuracy in the operation of the operational amplifier with respect to its theoretical characteristics.
To alleviate this drawback, FIG. 3 shows an exemplary operational amplifier comprising architecture similar to that of FIG. 1, and a device 9 for eliminating the offset voltage, allowing the zeroing of the offset voltage Voffset. FIG. 4 represents in greater detail the electrical circuit of the device 9 for eliminating the offset voltage according to the prior art, disposed at the level of the differential input stage 6. It is based upon a current source 10, the current provided Ib being independent of temperature variations, known by the simplified term of ZTAT (Zero to Absolute Temperature). This circuit comprises n equivalent first resistors Rp (R1p to Rnp) arranged in series from the source of the first transistor M1 attached to the first input 2 of the operational amplifier, and separated by various intermediate contacts T1p to Tnp on which a resistor switching contact is possible. In a similar manner, n equivalent second resistors Rm (R1m to Rnm) are arranged in series with the previous resistors (R1p to Rnp) up to the source of the second transistor M2 attached to the second input 3 of the operational amplifier. These n second resistors are separated by various intermediate contacts T1m to Tnm on which a resistor switching contact is possible. A contact T0 is arranged between these two sets of resistors Rp and Rm. The various contacts Tip and Tjm may be linked to the current source 10 by way of a switchable contact 11. The positioning of this contact makes it possible to ultimately define the resulting overall resistors R1, R2 arranged on either side of this contact 11 and therefore of the current source 10.
FIG. 4 illustrates the particular case where the switchable contact 11 is positioned on the central contact T0. The various voltages represented in FIG. 4 are governed by the following relation:Vs=Vp−Vgsm1−n*Rp*Ib/2=Vm−Vgsm2−n*Rm*Ib/2.By calling VRp=n*Rp*Ib/2 and VRm=n*Rm*Ib/2, the above equation becomes:Vs=Vp−Vgsm1−VRp=Vm−Vgsm2−VRm. 
By taking account of the offset voltage Voffset, the voltage Vp becomes Vp−Voffset and the above equation ultimately gives:Vp−Voffset−Vgsm1−VRp=Vm−Vgsm2−VRm.  (1)By modifying the positioning of the switchable contact 11 of the circuit of FIG. 4, for example, by displacing it by k contacts to the left in the case where the offset voltage is positive, the above equation becomes:Vp−Voffset−Vgsm1−(VRp−kRpIb/2)=Vm−Vgsm2−(VRm+kRp*Ib/2).This equation can also be written:Vp−(Voffset−2*kRpIb/2)−Vgsm1−VRp=Vm−Vgsm2−VRm  (2)
By comparing this equation (2) with equation (1), it is therefore realized that the switching of the switchable contact 11 has an effect equivalent to a reduction in the offset voltage by a value 2*k Rp Ib/2. Thus, it is possible to choose k in such a way as to obtain the lowest possible value of the offset voltage and this approach makes it possible to eliminate or reduce this offset voltage.
Naturally, the same principle is applied if the offset voltage is negative, in which case the contact 11 is switched to the right, and makes it possible to increase this offset voltage by a value of 2*k Rm Ib/2. This discrete value, used to eliminate the offset voltage, will be chosen as low as possible. With the use of the current source of ZTAT type to induce the current Ib, the variation of this quantity with temperature is minimized.
Though this approach may be effective in reducing the offset voltage of an operational amplifier in a temperature-independent manner, it may exhibit the drawback, however, of greatly reducing its performance at the level of its gain-bandwidth product. Indeed, as is apparent in FIG. 5, which represents the curve 15 of the gain-bandwidth product of the amplifier with temperature, this gain-bandwidth product drops by 40% over a temperature span from −40 to 125° C. It is noted that the gain-bandwidth product of an operational amplifier implementing the approach described hereinabove operates in a manner substantially proportional to the inverse of the absolute temperature (1/T).